Process for the manufacture of gypsum board

ABSTRACT

A process for the manufacture of a gypsum board wherein Alpha type hemihydrate gypsum by-produced in the wet process of phosphoric acid manufacturing is ground and mixed with water and a metal ion of iron or aluminum to form an aqueous slurry containing said gypsum and said slurry is enclosed between two sheets of board paper while maintaining the pH value of said slurry at 4 to 7, thereby said slurry sets to form said gypsum board.

llnited States Patent [1 1 Omoto et a1.

PROCESS FOR THE MANUFACTURE OF GYPSUM BOARD Inventors: Tsunehiko Omoto;Tadashi Inoue; Hironori Aizawa, all of Shimonoseki, Japan Mitsui ToatsuChemicals, Incorporated, Tokyo, Japan Filed: Oct. 4, 1971 Appl. No.:186,487

Assignee:

Foreign Application Priority Data Oct. 15, 1970 Japan 45/90065 US. Cl.106/110, 156/39 Int. Cl. C04b 11/00 Field of Search 106/109, 110, 111;156/39 References Cited UNITED STATES PATENTS 8/1965 Johnson 106/109 1Nov. 20, 1973 King 106/110 Janninck 106/110 Primary Examiner-James E.Poer AttorneyAmold B. Christen et a1.

[5 7 ABSTRACT 5 Claims, No Drawings 1 PROCESS FOR THE MANUFACTURE OFGYPSUM BOARD This invention relates to an improved process formanufacturing a gypsum board from a-type hemihydrate gypsum which isby-produced in the wet process of phosphoric acid manufacturing.

Hitherto, there has been used as a starting material for manufacturinggypsum board dihydrate gypsum byproduced in the manufacture ofphosphoric acid other than natural gypsum. The by-produced dihydrategypsum is calcined at a temperature of 120 200 C to convert into acalcined gypsum, then ground finely up to a specific surface area morethan 5,000 cm /g (Blaine value) and mixed with additives such as loadingmaterials, blowing agent etc. and a suitable amount of water to form aslurry. The slurry is enclosed between two sheets of board paper. Theresulting product is allowed to set, and, thereafter, it is dried toyield a gypsum board. The calcined gypsum formed in the above process isB-type hemihydrate having a porous structure, can be readily groundfinely and has a large conversion velocity into dihydrate in the aqueousslurry in the process for the manufacture of gypsum board.

In the wet process of phosphoric acid manufacturing, such as thedihydrate-hemihydrate process which has been developed recently, calciumphosphate rock is decomposed with a mixture of sulfuric and phosphoricacid, the formed dihydrate gypsum is separated from the formedphosphoric acid, sulfuric acid or a mixture of sulfuric acid andphosphoric acid is added to the separated dihydrate gypsum, theresulting slurry is kept at a temperature above the transition point ofdihydrate to hemihydrate in the presence of crystal seeds of hemihydrategypsum to convert dihydrate gypsum contained in said slurry intohemihydrate gypsum, the hemihydrate gypsum is separated from thesulfuric acid-phosphoric acid mixture and the mixture is used for thedecomposition of phosphate rock, a-type hemihydrate gypsum having l/2mol of crystallization water in a molecule instead of dihydrate gypsumis separated from the phosphoric acid solution.

If such a-type hemihydrate gypsum can be used for the manufacture ofgypsum board as it is, the calcination step of dihydrate gypsum can beomitted which is understood as inevitable in the manufacture of gypsumboard previously. However, the a-type hemihydrate gypsum containsimpurities such as phosphoric acid and fluorides and has a compactcrystalline structure, so that it has a small setting velocity and cannot be used for the industrial production of gypsum board as it is.

For the industrial production of gypsum board, it is general to add asuitable amount of water to B-type hemihydrate gypsum with stirring,enclose the resulting aqueous slurry between two sheets of board paperrunning continuously on a belt conveyer and combine with the coversheets by means of setting (dihydration) of hemihydrate gypsum for avery short time, e.g., l 7 min. Accordingly, the small setting velocityof a-type hemihydrate gypsum by-produced in the wet process ofphosphoric acid manufacturing is a fatal disadvantage from theindustrial view-point for the use in the manufacture of gypsum board;that is, there takes place such a fatal disadvantage that the reductionof speed of belt conveyer in the industrial production of gypsum board,

i.e., a remarkable reduction in the production capacity of gypsum board,may be brought about.

An object of this invention is to provide a process for the manufactureof gypsum board without the calcination step of dihydrate gypsum.

Another object of this invention is to provide an improved process forthe manufacture of gypsum board with a-type hemihydrate gypsum, which isby-produced in the manufacture of phosphoric acid, at a highmanufacturing velocity.

Further object of this invention is to provide a process for themanufacture of gypsum board without finely grinding starting a-typehemihydrate gypsum.

Still further object of this invention is to provide an improved processfor the manufacture of gypsum board with saving heat for drying.

According to this invention, the following process for the manufactureof gypsum board is provided. In a process for the manufacture of gypsumboard wherein a-type hemihydrate gypsum by-produced in the wet processof phosphoric acid manufacturing is ground and mixed with water to forma slurry and said slurry is enclosed between two sheets of board paperto set therebetween, the improvement which comprises making iron oraluminum ion exist in said slurry and maintaining the pH-value of saidslurry at 4 to 7.

For making iron or aluminum ion exist in the slurry, a water solubleiron or aluminum salt is mixed with a-type hemihydrate gypsum prior tothe formation of slurry containing a-type hemihydrate gypsum or added toan aqueous slurry containing a-type hemihydrate gypsum. As said watersoluble iron or aluminum salt, there is used a salt of strong acid, e.g.sulfate, nitrate or chloride, and sulfate is most preferable. The amountof iron or aluminum salt added is preferably 0.3 3.0 parts by weight ashydrate to parts by weight of a-type hemihydrate gypsum.

For the adjustment of pH-value of the aqueous slurry containing a-typehemihydrate gypsum to 4 to 7, preferably 4 to 6, a basic compound ofalkali metal or alkaline earth metal is added to the starting a-typehemihydrate gypsum or to the slurry. As said basic compound of alkalimetal or alkaline earth metal, there is used oxide, hydroxide orcarbonate thereof and most preferably calcium compound such as calciumhydroxide. The amount of said compound added is in general preferably0.05 0.5 part by weight per 100 parts by weight of a-type hemihydrategypsum.

In this invention, it is important to present iron or aluminum ion inthe aqueous slurry of a-type hemihydrate gypsum and keep the pH-value ofthe slurry at 4 to 7. As described hereinafter, only either the presenceof iron or aluminum ion or the adjustment of pH-value of the slurry doesnot give any effect or industrial utility. That is, if iron or aluminumion is present and the pH-value of slurry is not adjusted, the pH-valueof the aqueous slurry is lowered further in connection with the factthat the pH-value of gypsum by-produced in phosphoric acid manufacturingis less than 3, causing the changing of color and the weakening of boardpaper and the reduction of strength of gypsum board itself. Further,nails for fitting rust disadvantageously in the use of gypsum board.

On the other hand, if the pH-value is adjusted to 4 to.

7 in the absence of iron or aluminum ion, as obviously seen from theexamples described hereinafter, the setting time of the aqueous slurryof a-type hemihydrate gypsum is greatly prolonged with the loss ofadhesion to board paper and the reduction of strength. Potassium orsodium salts such as potassium or sodium sulfate, which have been usedconventionally in the manuwith B-type hemihydrate gypsum is notsubjected to the fine grinding and the setting velocity of a-typehemihydrate gypsum slurry is higher than that of conventional B-typehemihydrate gypsum slurry. Both the shortening facture of gypsum boardfrom B-type hemihydrate gyp 5 of setting time and the reduction of theamount of sum, do not provide effects as in this invention even if waterused, said reduction being resulted from the use they are usedrespectively singly or after adjusting the of a-type hemihydrate gypsumin the manufacture of pH-value of aqueous slurry within the range of 4to 7 gypsum board, bring a large increase of production caas in thisinvention. paeity per unit equipment. Further, the gypsum board in thisinvention, it is not necessary to grind finely the 10 obtained by thisinvention shows a superior adhesion of starting a-type hemihydrategypsum to such an extent core gypsum to board paper and bending strengthto as in conventional processes. While B-type hemihythose of gypsumboard obtained by the conventional drate gypsum, which has been usedpreviously, is finely process using B-type hemihydrate gypsum. ground ingeneral up to a specific surface area more According to this invention,the setting time of slurry than 5,000 cmlg, it is sufficient accordingto the proof a-type hemihydrate gypsum by-produced in the cess of thisinvention to grind the starting a-type gypmanufacture of phosphoric acidmay be equivalent to sum up to a specific surface area more than 2,500that in the use of B-type hemihydrate gypsum, so that cmlg, preferablyof about 3,000 cm lg. the equipments for manufacturing gypsum boardusing 1n the process according to this invention, there may B-typehemihydrate gypsum can be used in this invenbe used additives such asknown loading materials and tion as they are. blowing agents, which areused in the aqueous slurry in Examples according to this invention aredescribed the manufacture of gypsum board conventionally. as follows, inwhich parts and per cents are shown by According to this invention,a-type hemihydrate gypweight. sum by-produced in the wet process ofphosphoric acid Example 1 manufacturing such as thedihydrate-hemihydrate pro- A dried a-type hemihydrate gypsum by-producedin cess can be used very readily from the industrial viewthe wet processof phosphoric acid manufacturing acpoint for the manufacture of gypsumboard, so that the cording to the dihydrate-hemihydrate process was hightemperature calcination step (calcined gypsum used. The a-typehemihydrate gypsum contained 6.43 manufacturing step) of the startinggypsum is not repercent of crystallization water, 0.15 percent of totalquired which has been previously understood as ineviphosphoric acid (P 00.09 percent of water soluble table in the manufacture of gypsum board.phosphoric acid (P 0 0.11 percent of fluorine and Further, while waterfor normal consistency (parts by 0.10 percent of aluminum (M 0 Thea-type hemihyweight of water amount per 100 parts by weight of drategypsum was ground up to a specific surface area hemihydrate gypsumrequired for the preparation of of about 3,000 cm /g and tested followedby the test aqueous slurry of suitable consistency) of B-type heminumberas shown in the Table 1. hydrate gypsum is about 75 to 85, that ofa-type hemi- The ground a-type hemihydrate gypsum was mixed hydrategypsum is about 55 to 60. Therefore, the use strongly with 0.5 percentof dextrin (the addition of a-type hemihydrate gypsum can reduce aboutby 20 amount shown hereinafter is indicated in per cent by to 30% theamount of water required for the preparaweight referred to the a-typehemihydrate gypsum), tion of slurry of suitable consistency.Accordingly, the 0.05 percent ofsodium dodecylbenzenesulfonate, basicamount of water to be evaporated from the gypsum compound of alkalimetal or alkaline earth metal and board moldings in the drying step ofgypsum board is water soluble iron or aluminum salt in an amount asreduced about by 30 to 40 percent resulting in an imshown for each testnumber respectively in the table 1 provement of production capacity ofgypsum board and 60 parts of water per 100 parts of said a-typehemiplant. hydrate gypsum for 20 seconds in a pin-mixer at a rota-Further, in this invention, a-type hemihydrate gyption speed of 750r.p.m., the resulting slurry was taken sum which is very diffic ultlyfinely ground compared out to form a gypsum board of 9 mm thick by meansTABLE 1 Additives Slurry test Test of gypsum board moldings Watersoluble iron Setting time Bending Basic compound or aluminum (min-sec.)Adhesion strength compound g./cm.2)

Per- Per- Initi- Tenni- Sur- Under- Verti- Hori- Test N0 Kind cent Kindcent pH ation nation face surface cal zonta1 Note 1 Not used Not used3.1 4-10 10-20 3 7 62 22 2 Ca(0H)z 0.30 do 6.3 13-20 21-30 1 s Controls.8(OH): 0. 07 Fez(SO4)a 1.0 3.0 3-50 0-15 0 0 23 Ca(OH)g 0.12 ransom 1.04.5 1-15 2-15 3 0 62 28 838% 81. .3 @283; 1:8 233 i 38 3213 3 3 2% 33Examples mdmg 151mm Oa(OH)- 0.42 Feg(SO 1.0 8.1 5-10 10-00 6 8 55 24among 0.02 FG2(SO4)3 1.0 9.8 10-20 21-10 0 7 niolmtmle ggg g 2 }Examplesaccording to this invention. 11 Na=003 0.22 Ch 1.0 4.5 1-45 3-00 0 0 0320 No'rE.-In the Table 1: (1) The percent is referred to a-typehemihydrate gypsum; (2) The aqueous F02(SO4)3 having a content of l o(SOt)a, Al:(SOr)3 18 1120 and A1013 01 guaranteed grade were used; (3)the setting time was determined according to the specification ofJapanese Industrial Standard Method (J 18 R9112); the initiation time(shown as initiation) is a time required from the beginning of thepreparation of slurry to the time when the top of Vicat needle isstopped at a height of 1 mm. from the bottom of sample and the apparenttermination time (shown as termination) is a time required from thebeginning of the preparation of slurry to the time when the top of Vicatneedle is stopped at a depth of 1 mm. from the surface of sample: (4)the adhesion in each test number is a mean value of three test sampleswhich were cut from gypsum board moldings and tested according to thespecification of Tapanesc Industrial Standard Method (JIS A6901) and thevalues shown in the column of adhesion for each test number are thelengths of parts of board paper (front and back surface) to the gypsumcore where the length of break surface is 9.

of mold for the gypsum board test (40 cm X 50 cm), the adhesion andbending strength of moldings were determined and the results were shownin the table 1. The pH-values and setting times of said slurries werealso given in the table.

As obviously seen from the table 1, according to this invention, theslurry setting time of a-type hemihydrate gypsum shows an initiationtime of l to 3 minutes, more especially 1.5 to 2.5 minutes, and anapparent termination time of 2 to 6 minutes, more especially 2.5 to 4.5minutes, these values being industrially preferably for the manufactureof gypsum board. The gypsum board prepared according to this inventionshows also good values in the adhesion and bending strength.

Example 2 The same a-type hemihydrate gypsum as used in Example l wasground respectively up to the grinding degree as shown in the followingtable 2, the ground a-type hemihydrate gypsum was mixed respectivelywith 0.5 percent of dextrin, 0.27 percent of calcium hydroxide, 0.5percent of iron sulfate and 60 parts of water per 100 parts of saida-type hemihydrate gypsum for 20 seconds in a pin-mixer at a rotationspeed of 750 r.p.m. and the pH-value and setting time of the resultingaqueous slurry were determined respectively.

TABLE 2 Specific surface area 7 Setting time of a-type (min-sec.)hemihydrate gypsum Termina- Test N (cmJ/g.) pH Initiation tion Asobviously seen from the results in the table 2, if a-type hemihydrategypsum by-produced in the manufacture of phosphoric acid is ground up toa specific surface area above 2,500 cm /g according to this invention, aslurry is obtained which has a preferred setting time for themanufacture of gypsum board.

What is claimed is:

1. A process for the manufacture of a gypsum board which comprisesforming an aqueous slurry of a-type hemihydrate gypsum directly asobtained as by-product in the wet dihydrate-hemihydrate process ofmanufacturing phosphoric acid, in the presence of a metal ion selectedfrom the group consisting of iron and aluminum ions, maintaining the pHof said slurry at a value of 4 to 7, and enclosing said slurry betweentwo sheets of paper to form said gypsum board.

2. A process as claimed in claim 1 wherein a watersoluble salt of ametal selected from the group consisting of aluminum and iron is addedas said metal ion to said hemihydrate gypsum in an amount of 0.3 to 3.0parts by weight as hydrate of said salt per parts by weight of saida-type hemihydrate gypsum.

3. A process as claimed in claim 1 wherein a watersoluble salt of ametal selected from the group consisting of aluminum and iron is addedas said metal ion to said slurry in an amount of 0.3 to 3.0 parts byweight as hydrate of said salt per 100 parts by weight of said a-typehemihydrate gypsum.

4. A process as claimed in claim 1 wherein said pH value is maintainedby adding a member selected from the group consisting of basic compoundsof alkali metals and alkaline earth metals to said slurry.

5. A process as claimed in claim 1 wherein said a-type hemihydrategypsum is ground to a specific surface area between 2,500 and 5,000cmlg.

2. A process as claimed in claim 1 wherein a water-soluble salt of ametal selected from the group consisting of aluminum and iron is addedas said metal ion to said hemihydrate gypsum in an amount of 0.3 to 3.0parts by weight as hydrate of said salt per 100 parts by weight of saidAlpha -type hemihydrate gypsum.
 3. A process as claimed in claim 1wherein a water-soluble salt of a metal selected from the groupconsisting of aluminum and iron is added as said metal ion to saidslurry in an amount of 0.3 to 3.0 parts by weight as hydrate of saidsalt per 100 parts by weight of said Alpha -type hemihydrate gypsum. 4.A process as claimed in claim 1 wherein said pH value is maintained byadding a member selected from thE group consisting of basic compounds ofalkali metals and alkaline earth metals to said slurry.
 5. A process asclaimed in claim 1 wherein said Alpha -type hemihydrate gypsum is groundto a specific surface area between 2,500 and 5,000 cm2/g.